1. Field of the Invention
The invention relates to the manufacture of mufflers for the exhaust systems of internal combustion engines, and provides an assembly technique and structure that substantially eliminates the break-in or seasoning period associated with conventionally constructed new mufflers. A mass of fibrous muffler packing, especially long or preferably continuous-strand fiberglass, is restrained in size and shape during its permanent enclosure between the walls of a muffler housing, by an open mesh enclosure for facilitating handling and assembly. The mesh preferably comprises a consumable flexible enclosure comprising open net polymer material with heat sealed seams. Alternatively, a similar preparatory enclosure for confining and shaping the packing during assembly can be made from fiberglass, steel, aluminum or copper screening or mesh, or from another open mesh material such as a textile fiber.
The flexible enclosure holds or compresses the packing to clear the seams between the walls and/or internal plenums of the muffler housing, especially during welding. The enclosure also holds the loose fiber in a pack, so that it can be dropped in easily as a unit during muffler assembly. In addition to facilitating the handling of the long strand fiberglass packing, the open net form of the enclosure material has little or no acoustic effect. Therefore, the sound or "tune" of the muffler upon initial operation of the engine is substantially the same as its tune long after the polymer has melted or burned away.
2. Prior Art
The primary function of an exhaust muffler is to mute the noise produced by the cylinders of an internal combustion engine. This is accomplished by placing a sound absorbing muffler packing, typically comprising fibers, in at least one cavity in a muffler housing disposed along the exhaust path. Sound vibrations are absorbed in the circuitous paths between the fibers and by resilient deflection of the fibers, which typically comprise a flexible heat and corrosion-tolerant material such as fiberglass.
Mufflers do not reduce the emitted engine noise to zero. A muffler is structured so as to have a desired effect on the tonal qualities of the sound emitted by the engine. Some mufflers, for example those of performance automobiles, are deliberately made louder or lower pitched than others, for example those of luxury cars. Resonances due to the size and shape of flowpaths defined by walls and plenums through the muffler housing, the path of the exhaust gases through the housing, and as modified by the packing material, are exploited to obtain the desired sound.
A conventional muffler housing has a generally cylindrical external shape, or for mounting under the vehicle chassis often is flattened to have an oblong cross section or a flattened cylindrical shape for better vertical clearance. In manufacturing mufflers, it is necessary to form and to assemble the necessary external and internal walls and plenums, and to assemble them together with the muffler packing so that the muffler packing is confined within the walls.
In a typical muffler construction, inlet and outlet tubes are coupled to opposite ends of a housing. Within the housing, the tubes and the housing walls define various chambers leading generally in a zigzag path whereby exhaust gases circulate in the muffler. At least one chamber or tube is disposed adjacent to or within another chamber or tube with one or more perforated walls. Packing material is compressed into one of these chambers or tubes and the other defines a gas flow path.
Advances in muffler design have included a muffler housing with shallow stamped upper and lower elongated bowl shaped half shell members and one or more internal plates or walls dividing the internal volume, for example substantially along the seam between the upper and lower members. The seam can be welded and/or formed by bending flanged edges of the upper and lower members over one another. A circuitous path is defined between the internal wall and the upper and lower members, with fibrous packing compressed between the internal wall and the upper or lower member on the respective side. Stamped plate mufflers are available from AP Parts Manufacturing Company of Toledo, Ohio. Such mufflers are disclosed, for example, in U.S. Pat. Nos. 4,700,806--Harwood; U.S. Pat No. 4,759,423--Harwood et al.; 4,860,853--Moring III; 5,428,194--Emrick et al.; and, 5,563,385--Harwood, which are hereby incorporated. The present invention is applicable to such stamped plate mufflers and also to more traditional structures, for example having a perforated pipe passing through a cylindrical external housing wall having a tubular chamber confining packing between the perforated tube, the outer wall and annular end plates.
Assembly steps involving stamped plate mufflers can require placement of the packing material in one or more of the wall or plenum structures and compression of the packing into a particular area and shape when attaching other wall or plenum structures. For example, in a stamped plate muffler the packing may be placed in a portion of one half shell adjacent to a gas flow path and compressed under an intermediate plate. The assembly is then flipped over onto the other half shell, which may also have packing placed at a particular chamber therein. It can be difficult to effect such a procedure while properly placing the packing material and without the packing material interfering with the respective seams as it is compressed.
A good material for muffler packing is fiberglass strands. Preferably, long strand or even continuous strand fiberglass is shaped into a mass and compressed as necessary to complement the housing cavity. Strand material is readily restrained from migration into the exhaust stream by one or more panels crossing the flowpath or bounding the flowpath, having spaced holes, slots or other openings to allow gas flow. For long strand loose packing, relatively large holes and slots are sufficient to restrain the packing because they are still small in comparison to the length of the strands and in comparison to any loops which protrude from the packing mass.
Nevertheless, loose strand packing material can be difficult to shape properly and to distribute evenly in the housing cavity. One method for assisting in assembly is to preform the packing into a fixed shape that is complementary with the housing cavity or portion of a housing cavity in which the packing is to reside. For example, a hardening resin can be applied to the packing, which is then pressed or molded into the required shape. The resin bonds the fibers at points where they touch and/or cross one another. When the resin is cured, the packing holds its shape. The fixed shape of the packing is such that outer walls of the muffler can be attached over the packing and the packing does not tend to come between the parts and interfere with the formation of seams.
Another option for containing fibrous packing is to over-wrap the packing on a structural element of the muffler, for example with strapping, plastic sheet or the like. The packing is later covered by an outer structural element. The strapping outwardly confines the packing and prevents interference with seams and the like.
Resin used in preforming the packing, and wrapping material confining loose packing, each result in material other than the packing (namely the confining or shape-holding material) being left in the finished muffler. This material affects the operation of the muffler during initial operation on a vehicle. The confining material can be made to burn away, but nevertheless affects the muffler until it burns away completely to leave the non-consumable fiberglass or other packing. The effect on operation is due to the fact that available flowpaths change over time and the physical characteristics of the fibrous packing, such as the flexibility of the fibers and their freedom to move, change over time. As a result, the sound or tune of a new muffler having such confining material changes over time. Assuming that the muffler is designed to sound best once the confining material burns away, the muffler necessarily is not tuned optimally at the beginning of operation.